A hemispherical power asymmetry from inflation

Measurements of cosmic microwave background temperature fluctuations by the Wilkinson Microwave Anisotropy Probe indicate that the fluctuation amplitude in one half of the sky differs from the amplitude in the other half. We show that such an asymmetry cannot be generated during single-field slow-roll inflation without violating constraints to the homogeneity of the Universe. In contrast, a multifield inflationary theory, the curvaton model, can produce this power asymmetry without violating the homogeneity constraint. The mechanism requires the introduction of a large-amplitude superhorizon perturbation to the curvaton field, possibly a preinflationary remnant or a superhorizon curvaton-web structure. The model makes several predictions, including non-Gaussianity and modifications to the inflationary consistency relation, that will be tested with forthcoming cosmic microwave background experiments.

Figure 1

Measurements of temperature fluctuations in the CMB show that the rms temperature-fluctuation amplitude is larger in one side of the sky than in the other. We investigate here whether this may arise as a consequence of a large-scale mode of an inflaton or a curvaton.

Figure 2

The $R$-$\xi$ parameter space for the curvaton model that produces a power asymmetry $A=0.2$ (top panel) and $A=0.05$ (bottom panel). Here $R$ is the fraction of the cosmological density due to curvaton decay, and $\xi$ is the fraction of the power due to the curvaton. The upper limit to $R$ comes from the CMB-quadrupole constraint. The lower bound comes from $f_{\mathrm{NL}}\le 100$. The lower limit to $\xi$ comes from the requirement that the fractional change in the curvaton field across the observable Universe be less than 1. If $A$ is lowered, the lower bound to $R$ remains unchanged, but the upper bound increases, proportional to $A^{-2}$. The lower limit to $\xi$ also decreases as $A$ decreases, proportional to $A$.